1. Field of the Invention
This invention relates to a low mercury arc discharge lamp containing a halide of praseodymium. More particularly this invention relates to a high intensity, electrodeless arc discharge lamp wherein the arc tube contains an essentially mercury-free fill including a halide of praseodymium.
2. Background of the Disclosure
High intensity electroded arc discharge lamps such as high pressure sodium lamps and metal halide lamps are well known and include a light-transmissive arc discharge chamber or tube hermetically sealed and enclosing within a pair of spaced apart electrodes and a suitable fill such as an inert starting gas and one or more ionizable metals or metal halides. Two of the major causes of lamp failure are sputtering of electrode material onto the lamp envelope and thermal and electrical stresses which result in electrode failure. More recently a new class of arc discharge lamps has been developed called electrodeless lamps. Such lamps have a light-transmissive, electrodeless arc chamber or tube generally shaped like a pillbox or slightly flattened sphere and containing a fill which comprises a suitable inert buffer gas and one or more metal halides. Radio frequency (RF) energy applied or coupled to the fill via capacitive or inductive coupling generates a light-emitting arc. In operation of such a lamp via inductive coupling, the arc tube or chamber acts as a single-turn secondary coil of a transformer and is surrounded by an RF energy excitation coil which acts as a primary coil. Various embodiments of such lamps are disclosed, for example, in U.S. Pat. Nos. 4,810,938; 4,972,120; 4,959,584 and 5,039,903 all of which are assigned to the assignee of the present invention. Continuing research and development has been directed towards improving the color emitted by electrodeless arc discharge lamps while maintaining the relatively high color rendering index (CRI) and lamp efficacy exhibited by these lamps.